Steam has received an update, boasting a new look and a bunch of features, one of which is the neat-sounding Guest Pass:

Guest Passes allow owners of certain Steam games (purchased either via Steam or at retail) to let their friends play games for free, for a limited time trial. The first game to take advantage of this new feature is Day of Defeat: Source. Over the next few days, owners of DoD:S will each receive a guest pass to give to a friend. So if you own Day of Defeat: Source, watch your games list for your guest pass to appear, and then send it to a friend.

It will now also download stuff in the background, so if you're on a limited bandwidth plan you might want to check into that.